Trivalent rare earth ions efficiently emit luminescence in amorphous silicon based alloy hosts. The luminescence details depend mostly on the immediate chemical neighborhood of the ions. In crystals the luminescence peaks at low temperatures can be adjusted by lorentzian lineshapes with energies determined by the symmetry of the well defined lattice sites. In disordered materials however not only the local symmetry can change but also the local stoichiometry is not well defined. As a result the luminescence peaks become gaussian in shape. Nevertheless, by carefully adjusting the luminescence spectra information can be obtained about the local properties. We studied the luminescence of Eu3+ and Tb3+ in amorphous silicon alloys at low temperatures. The results indicate that Eu3+ is mostly on distorted cubic symmetry sites, while for Tb3+ it is not possible to determine the symmetry reliably. In any case, the results are compatible with the rare earth ions coordinated to residual oxygen in the alloys.